In order to increase the clinical utility of the CEA assay for the diagnosis and monitoring of malignant disease, it is necessary to characterize CEA chemically and to identify its determinants. Since our research strongly suggests that the antigenic determinants measured in the radioimmunoassay resides in the protein portion of the molecule, emphasis will be given to the isolation and characterization of peptides obtained from CEA by specific enzymatic and chemical cleavage. N-Terminal amino acid sequences will be determined on the isolated peptides. It will be attempted to raise antisera against specific peptides and their synthetic N-Terminal peptide analogues. Peptides and antisera raised against them will be tested as reagents in the CEA radioimmunoassay. Similar studies are in progress on several CEA-related antigens, since an understanding of their chemical relationships to CEA is necessary to improve the specificity of the CEA radioimmunoassay.